1. Field of Invention
The present invention relates to a power output device, and more particularly, to a power output device with protection function for short circuit and overload.
2. Description of the Related Art
A conventional power output device usually uses a transistor or a plurality of transistors to form a bridged output stage for amplifying a signal and outputting the amplified signal to a load. An inadvertent operation, however, may cause an abnormally large current flowing through the transistors and thus damaging the transistors. For example, load is in short circuit, and the output terminal is short to ground or to a voltage source, all such foregoing conditions may easily damage the whole circuit and cause danger. For the reason to prevent the power output device being out of order due to the aforementioned abnormal operation, and thereby improve reliability of circuit, a protection circuit for short circuit and overload is designed to protect the power output device.
FIG. 1 is a conventional power output device with a protection circuit for short circuit and overload. Referring to FIG. 1, the power output device comprises an input control unit 11 for receiving a signal, a driving circuit 12 for receiving the output signal from the input control unit 11, a power output unit 13 driven by the driving circuit 12 for outputting an amplified signal to a load 10, a resistor 14 connected in series with the load 10, a reference voltage source 15 and a comparator 16. The power output unit 13 can be constructed by a transistor. The output current from the transistor flows through the load 10 and the resistor 14, on which a voltage is generated between two terminals of the resistor 14. The generated voltage drop is inputted into a positive input terminal of the comparator 16, while the negative input terminal of the comparator 16 is connected to the reference voltage source 15. Thus, the output current from the transistor is converted into a voltage signal. Through comparing the voltage signal with the output voltage of the reference voltage source 15, if short circuit or overload occurs, an abnormal large current can be detected.
If the output current is normal (that is, the output current is within a normal range), the voltage of the resistor 14 is less than a reference voltage value outputted by the reference voltage source 15. At this moment, the output voltage of the comparator 16 is in a low level, and the input control unit 11 controls the transistor to normally operate and provide current to the load. If the output current is abnormally large due to short circuit or overload, the voltage of the resistor 14 becomes larger than a reference voltage value. The output voltage of the comparator 16 immediately rises to a high level, so as to drive the input control unit 11 to turn off the transistor via the driving circuit 12. Thereby, it can prevent the transistor from burning out by an over current.
However, although the aforementioned protection mechanism can achieve the object of protecting the output power device, it requires an element (such as the resistor 14 connected in series with the load 10) to provide a voltage for detecting. The resistor 14 consumes a certain power, it introduces lower power efficiency and reduces output power. Moreover, if the resistor 14 is integrated into an IC, the issue of thermal dissipation in IC package should be considered due to the resistor 14 generates additional heat.